Use of counter-rotating propellers is well known, offering the possible benefit of increased propeller system power density and elimination of power wasting swirl from the downstream prop wash. However, there are also adverse effects leading to higher noise levels and difficulty keeping both propellers on their intended operating design points during changing power and flight conditions. A significant problem with prior art counter-rotating propellers is the mismatch of the forward prop tip slipstream to the aft prop tip as it goes through the plane of the aft propeller. If the front prop tip slipstream (and prop wash) do not pass through the aft prop as desired, the aft prop performance and noise levels can be adversely affected. If the front prop tip slipstream passes outside the aft prop tip diameter, a partial prop wash outer swirl will persist downstream reducing potential prop set performance. If the front prop tip slipstream passes radially inward from the aft prop tip, the aft prop tip operates in an unintended region of low axial velocity free stream flow resulting in tip stall and corresponding high noise levels at low vehicle velocities and high prop power levels.
It is believed that no one prior to the inventor has made or used the invention described in the appended claims.